The invention relates generally to communication systems and more specifically to a communication system including multiple independent receivers.
FIG. 1 is a diagram illustrating a portion of a prior art cellular communication network. The cellular communication network includes base stations 101, 102, and 103. Base stations 101, 102, and 103 provide areas of coverage 104, 105, and 106, respectively, for voice communications. Base stations 101, 102, and 103 are located relative to each other to ensure complete coverage, even providing overlapping coverage in fringe regions of these areas of coverage 104, 105, and 106, such as xe2x80x9csoft handoffxe2x80x9d region 110.
However, while significant noise can be tolerated for voice communications, high speed data communications are more sensitive to noise. Therefore, high speed data communications require a higher signal-to-noise ratio than voice communications. Since signals become weaker as the distance between antennas increases, signals to and from base stations 101, 102, and 103 become weaker as a mobile unit moves farther from the base station. As the signals become weaker, the signal-to-noise ratio decreases. Since high speed data communications require higher signal-to-noise ratios than voice communications, transmission output power levels are increased in order to maintain the same area of coverage for high speed data communications as compared with analog communications. However, limitations on transmission output power normally prevent high speed data channels from maintaining the same area of coverage. Therefore, areas of coverage 107, 108, and 109 for base stations 101, 102, and 103, respectively, for high speed data communications are smaller than areas of coverage 104, 105, and 106 for voice communications.
Since the locations of many base stations were chosen for the purposes of voice communications, base stations 101, 102, and 103 are sometimes too far apart to provide seamless coverage for high speed data communications. For example, none of areas of coverage 107, 108, or 109 for high speed data communications include region 111. Thus, a mobile unit located in region 111 would be denied service for high speed data communications. Thus, a technique is needed to increase the reliability of high speed data communications and to allow uninterrupted high speed data communications across multiple base stations.
FIG. 6 is a block diagram illustrating a prior art receiver. A base station such as base stations 101, 102, and 103 includes such a receiver. The receiver includes antenna 601, demodulator and filter 602, automatic gain control (AGC) circuit 603, first despreader 604, nth despreader 605, channel correctors 606 and 607, deskewer/combiner 608, deinterleaver 609, and decoder 610. Antenna 601 is coupled to demodulator and filter 602. Demodulator and filter 602 is coupled to AGC circuit 603. AGC circuit 603:is coupled to a plurality of despreaders, illustrated by first despreader 604 and nth despreader 605. The despreaders are coupled to a plurality of channel correctors, illustrated by channel correctors 606 and 607. The channel correctors are coupled to deskewer/combiner 608. Deskewer/combiner 608 is coupled to deinterleaver 609. Deinterleaver 609 is coupled to decoder 610.
Decoder 610 provides a metric signal and data. Decoder 610 may be a Viterbi decoder. The metric signal provided by the decoder 610 is a correlation output of the most likely path chosen by the decoder from among many possible paths, which may be expressed in the form of a trellis diagram. This correlation output from the decoder of the most likely path chosen indicates the most likely data sequence based on the input to the decoder.
When a mobile unit is transitioning from an area of coverage of one base station to an area of coverage of another base station, the mobile unit operates in a xe2x80x9csoft handoffxe2x80x9d mode where the mobile unit communicates with more than one base station. For example, the mobile unit may communicate with three different base stations during a xe2x80x9csoft handoff.xe2x80x9d A xe2x80x9csoft handoffxe2x80x9d differs from a xe2x80x9chard handoffxe2x80x9d in that, for a xe2x80x9chard handoff,xe2x80x9d a mobile unit is in communication with only one base station at any given time, and the transition from one base station to another occurs at a specific moment in time. An example of a xe2x80x9csoft handoffxe2x80x9d process begins with a mobile unit communicating with a first base station within the area of coverage of the first base station. As the mobile unit moves toward a second base station, the mobile unit enters a region of xe2x80x9csoft handoffxe2x80x9d where the mobile unit is able to communicate with both the first base station and the second base station. If the mobile unit continues away from the first base station, the mobile unit leaves the region of xe2x80x9csoft handoffxe2x80x9d and remains in communication with the second base station.
The mobile unit transmits a reverse link signal to the base stations with which it communicates. To receive the reverse link signal transmitted by the mobile unit, each of these base stations attempts to decode the reverse link signal and sends its received frame data to a base station controller (BSC). Thus, the BSC receives the received frame data from each base station with which the mobile unit communicates.
FIG. 2 is a block diagram illustrating a prior art technique for determining a received datum from a plurality of data from a plurality of independent receivers. Base stations 201, 202, and 203 include receivers 204, 205, and 206, respectively. Each of receivers 204, 205, and 206 provides a metric signal and data to a base station controller 207. The base station controller 207 of the prior art functions as a multiplexer that simply chooses a frame of data from the base station with the largest metric signal. The base stations 201, 202, and 203 provide xe2x80x9chard decisionxe2x80x9d data to the base station controller 207. The xe2x80x9chard decisionxe2x80x9d data represent a determination by the base station as to what the final received data are. The xe2x80x9chard decisionxe2x80x9d data are independent of the metric signal and are independent of the xe2x80x9chard decisionxe2x80x9d data provided to the base station controller 207 by other base stations. Since the xe2x80x9chard decisionxe2x80x9d data involve a decision being made at a base station as to what the final received data are, the base station controller is merely able to select xe2x80x9chard decisionxe2x80x9d data from among that provided by the base stations.
The presence of multiple independent receivers provides what is referred to as diversity in receiving the reverse link signal from the mobile unit. The type of diversity where the base station controller 207 simply chooses the frame of data from the base station with the largest metric signal is referred to as selection diversity.
The receivers 204, 205, and 206 are independent receivers in that they are geographically separate from each other and they provide data over relatively low bandwidth links to a common location. The limited bandwidth of the links imposes some constraints on the manner in which the data are communicated.
One problem with the techniques relates to the difficulty of determining a signal-to-noise ratio of the signal carrying the data. The signal-to-noise ratio affects the likelihood that the data will be correctly interpreted. However, no information about the signal-to-noise ratio is typically transmitted from a base station to the base station controller. Consequently, no provision is made at the base station to determine the signal-to-noise ratio.
Even if circuits were added to a base station to determine the signal-to-noise ratio, such circuit would increase the cost and complexity of each base station in which they were used. With cell sizes being reduced and the number of base stations increasing, such additional cost and complexity of each base station would greatly increase the overall system cost. Moreover, even if such circuits were added, additional bandwidth would be required to communicate the signal-to-noise ratio information. Additionally, base station controllers are typically not equipped to handle such signal-to-noise ratio information.
FIG. 3 is a block diagram illustrating a prior art technique for determining a signal-to-noise ratio of a data signal associated with a datum. Convolution encoder 301 provides a signal that is impaired by noise added during communication through a medium 302. Thus, the signal present at Viterbi decoder 303 is impaired by the noise.
To determine the signal-to-noise ratio of the signal present at the input of the Viterbi decoder 303, the signal is passed to an averaging block 304 and to an adder 305. The averaging block determines an average of the signal over a long period of time and applies this average to adder 305 as a negative input. This average tends to cancel out the effects of noise, thereby leaving only the signal.
The adder adds the negative of the signal to the combination of the signal plus the noise, thereby yielding a noise output representative of the noise only. The noise output from adder 305 and the signal output from the averaging block 304 are applied to a divider 306, which divides the signal by the noise, yielding the signal-to-noise ratio.
However, dividers, such as divider 306, are typically complex and would increase the cost of a receiver. Moreover, once the signal-to-noise ratio is determined, it would need to be communicated meaningfully to the base station controller in a manner not currently provided.
Another prior art technique calculates an average bit error rate over a large number (e.g., 10,000) frames and relates this bit error rate to the signal-to-noise ratio. However, this technique does not provide the signal-to-noise ratio on a frame-by-frame basis, thereby reducing its effectiveness.
Another prior art technique for achieving diversity involves equal gain combining. This technique is usually used with xe2x80x9csoft decisionxe2x80x9d data since it does not take into account the metric signal. However, transmission of xe2x80x9csoft decisionxe2x80x9d data is usually not practical because of the large bandwidth required to do so. If an attempt were made to use this technique with xe2x80x9chard decisionxe2x80x9d data, it would not provide a way of distinguishing data on the basis of its reliability since it would not include an information as to reliability. Thus, this technique is not practical for use with geographically-distributed independent receivers.
Thus, a technique is needed that can determine a signal-to-noise ratio value, effectively communicate it, and meaningfully use it to improve the performance of a communication network.